This invention relates to zoom lenses for use with television cameras and movie cameras, and more particularly to zoom lenses having a large relative aperture with a range covering focal lengths used frequently.
The conventional practice is disclosed in Japanese Patent Sho 40-665 and U.S. Pat. No. 3,994,571. Three constituent lens components have, from the front, positive, negative and positive powers. When zooming from the wide angle to the telephoto mode the 2nd lens component is moved axially forward to change the focal length of the entire system, while the 1st lens component is simultaneously moved reciprocatingly in a convex path toward the front to maintain the position of a focal plane. When focusing, the 1st lens component is axially moved. Such a zoom lens is capable of reduction in bulk and size. In recent camera art, however, as the body becomes progressively more compact, a further minimization of the bulk and size of the camera including the lens system is desired. When the 3-component zoom lens is to be shortened in the longitudinal direction, the air separations between the successive lens lens components may be narrowed, and the axial thickness of each of the lens components may be reduced. The desired equivalent zoom ratio is present, however, by increasing the refractive power of each lens component with the production of steeply increasing aberrations in each lens component. Particularly difficult to remove are the spherical aberrations in the telephoto position and the variation of distortion with zooming.
The reduction of the imaging performance resulting from the increase of the refractive power of each lens component can be prevented to some extent by increasing the number of constituent lens elements. But, this results in the alternate disadvantage that the thickness and weight of the constituent lens component itself are increased. Thus, it is impossible to minimize of the bulk and size of the lens system as a whole, and, because of the increase of weight of the movable lens components, to improve the ease of management.
As another means of reducing the variation of the aberrations in the zooming range, the prior art has introduced an aspheric surface into the zoom lens. U.S. Pat. No. 3,972,592, relating to a 3-component varifocal lens for projectors in which the second component is moved to effect a change of the image magnification, has the aspheric provision in the 2nd component to correct spherical aberration. Also in U.S. Pat. No. 3,920,315 there is shown a varifocal lens for projectors too, in which the lens components, movable for zooming on either side of one stationary lens component, are each provided with an aspheric surface to correct spherical aberration. Furthermore, in U.S. Pat. Nos. 4,153,339, 4,159,865 and 4,196,968 there are shown 2-component zoom lenses with the introduction of an aspheric surface to correct spherical aberration in the telephoto position and distortion in the wide angle position.